Editing Vanguard 1 (section)

=== Design for atmospheric density measurements ===
Because of its symmetrical shape, Vanguard 1 was used by experimenters for determining upper atmospheric densities as a function of [[altitude]], [[latitude]], [[season]], and [[Solar phenomena|solar activity]]. As the satellite continuously orbited, it would deviate from its predicted positions slightly, accumulating greater and greater shift due to drag of the residual atmosphere. By measuring the rate and timing of orbital shifts, together with the body's drag properties, the relevant atmosphere's parameters could be back-calculated. It was determined that atmospheric pressures, and thus drag and orbital decay, were higher than anticipated, since Earth's upper atmosphere does taper off into space gradually.<ref name="Experiment2">{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/experiment/display.action?id=1958-002B-02|title=Satellite Drag Atmospheric Density|publisher=NASA|date=14 May 2020|access-date=1 February 2021}} {{PD-notice}}</ref>

This experiment was planned extensively prior to launch. Initial [[United States Naval Research Laboratory|Naval Research Laboratory]] (NRL) proposals for the project included conical satellite bodies; this eliminated the need for a separate fairing and ejection mechanisms, and their associated weight and [[separation event|failure modes]]. Radio-tracking would gather data and establish a position. Early in the program, optical tracking (with a [[Schmidt camera|Baker-Nunn camera]] network and [[Operation Moonwatch|human spotters]]) was added. A panel of scientists proposed changing the design to spheres, at least {{cvt|50.8|cm}} in diameter and hopefully {{cvt|76.2|cm}}. 

A sphere would have a constant optical reflection, and constant [[Drag coefficient|coefficient of drag]], based on size alone, while a cone would have properties that varied with its orientation. [[James Van Allen]] of the [[University of Iowa]] proposed a cylindrical satellite based on his work with [[rockoon]]s, which became [[Explorer 1]], the first American satellite. The Naval Research Laboratory finally accepted a sphere with a {{cvt|16.5|cm}} of diameter as a "Test Vehicle", with a diameter of {{cvt|50.8|cm}} set for the follow-on satellites. The weight savings, from reduced size as well as decreased instrumentation in the early satellites, was considered to be acceptable.

As Vanguard 1, [[Vanguard 2]], and [[Vanguard 3]] are still orbiting with their drag properties essentially unchanged, they form a baseline data set on the [[atmosphere of Earth]] that is over 60 years old and continuing.